1. Field of the Invention
The present invention relates to a power steering apparatus which uses an actuator to provide power to assist the steering force for a vehicle.
2. Description of the Related Art
Electric power steering apparatus have come into use for vehicles, wherein the steering apparatus uses, for example, an electric motor as an actuator to provide power to assist the driver's steering effort. In such electric power steering apparatus, the steering force is detected by a torque sensor and the electric motor is operated according to the torque detected by the torque sensor.
Also, the vehicle's speed is detected by a vehicle speed sensor, and control is performed so that when the detected speed is higher than a predetermined value, the operation of the electric motor is disabled, according to the torque detected by the torque sensor, to cut off the power assistance at high travelling speed, thereby enhancing the steering safety.
However, if the vehicle speed sensor fails, there arises a possibility that the electric motor to be operated according to the detected torque may not be disabled as intended; to avoid such a problem, a second vehicle speed sensor for fail-safe use is provided so that in case of failure of the first vehicle speed sensor, the electric motor can be disabled on the basis of the speed detected by the second vehicle speed sensor. When such a second vehicle speed sensor is provided for fail-safe use, the operation of the electric motor is usually disabled on the basis of the larger speed value of the two detected by the two vehicle speed sensors.
In a configuration in which the failure of the vehicle speed sensor is detected by monitoring the sensor-detected vehicle speed value over a short duration of time, if chattering occurs in the detected vehicle speed value, there is a possibility of erroneously detecting the vehicle speed, and once the erroneous vehicle speed value has been detected, the electric motor designed to provide power to assist the steering force will remain held in a disabled state even after the termination of the erroneous detection, thus decreasing the utilization rate of the electric motor. On the other hand, in a configuration in which the failure of vehicle speed sensor is detected by monitoring the detected speed values over a longer duration of time, there is a possibility that the electric motor may not be disabled until the sensor failure is detected, thus lowering the steering safety during high speed travelling.